SMTP

SMTP ports you need to understand are:

• TCP 25: default for non-encrypted communications.

• TCP 465: port registered by IANA for SMTP over SSL (SMTPS). Note, this has been deprecated in favour of STARTTLS.

• TCP 587: Secure SMTP (SSMTP) for encrypted communication using STARTTLS.

• TCP 110: default port for POP3 non-encrypted communications.

• TCP 995: default port for POP3 encrypted communications.

• TCP 143: default port for IMAP in non-encrypted communications.

• TCP 993: default port for IMAP in SSL/TLS communications.

Some useful SMTP commands for validation are:

• HELO: initiate SMTP conversation with mail server.

• EHLO: initiate ESMTP conversation with mail server.

• STARTTLS: start a TLS connection to mail server.

• RCPT: denote mail address of recipient.

• DATA: initiate transfer of contents of an email message.

• RSET: cancel an email transaction.

• MAIL: used to denote mail address of sender.

• QUIT: used to close connection.

• HELP: display help.

• AUTH: authenticate client to server.

• VRFY: used to verify if a user's mailbox exists.

• EXPN: request, or expand a mailing list.